When working with audio files that you exported from regions in Logic Pro 7.1, the Follow Tempo option may not always be available in the Region Parameter box (it will appear dimmed). This happens if the original region from which you exported the audio file doesn't start exactly on a quarter note beat.
For example, if you have a region that starts at a timeline position of 2 2 1 1 and export an audio file from it, the resulting file can be set to follow the tempo. However, if you have a region that starts at a timeline position of 2 2 3 1 and export an audio file from it, the resulting file won't be able to follow the tempo.
If you have a region that does not start on a quarter note, move it so that it starts on a quarter note beat before exporting the region. Alternatively, you can leave the region in its original position and export the entire track as an audio file by choosing File > Export > Track as Audio File. Then bring the resulting file back into the Arrange window to edit out any unwanted parts.